Democrats will "never" see US president Donald Trump's tax returns, said White House chief of staff Mick Mulvaney as a new front opened in the confrontation between the administration and Congress.

Mr Mulvaney accused Democrats of engaging in a "political stunt" and wanting "attention" after the chairman of the House Ways and Means Committee, Representative Richard Neal, asked the IRS (Internal Revenue Service) to provide six years of Mr Trump's personal tax returns and the returns for some of his businesses.

"That is not going to happen and they know it," Mr Mulvaney told Fox News Sunday.

Last week, Massachusetts Democrat Neal, one of only three congressional officials authorised to request tax returns, requested Mr Trump's personal and business returns in a letter to IRS Commissioner Charles Rettig.

He asked for returns covering 2013 through 2018. He also asked for the documents in seven days, setting an April 10 deadline.

An obscure 1924 statute includes no exceptions to Mr Neal's authority to ask the Treasury Department for returns and says the department "shall furnish" them when requested.

It does require that the review of an individual's return be conducted by members of Congress in "closed executive session" if the returns are provided without the taxpayer's consent.

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William Consovoy, whose firm was retained by Mr Trump to represent him on the matter, said in a letter Friday to the Department of Treasury's general counsel that the tax code zealously guards taxpayer privacy.

He said requests for tax returns "must have a legitimate legislative purpose".

Mr Neal has said he made the request to review the IRS policy of auditing the tax returns of all sitting presidents and vice presidents, saying, "little is known about the effectiveness of this programme".

"On behalf of the American people, the Ways and Means Committee must determine if that policy is being followed, and if so, whether these audits are conducted fully and appropriately," Mr Neal said when unveiling his request for Mr Trump's returns.

Utah Senator Mitt Romney, the Republican Party's 2012 presidential nominee who has been an occasional critic of Mr Trump's, called Democrats' actions "moronic".

He told NBC's Meet The Press that he believes Mr Trump should release his tax returns, but by demanding them, "I think the Democrats are just playing along his handbook."